1. Field of the Invention
The present invention relates to an electrode system for pacemakers, of the type having at least one ventricular electrode, intended for placement in contact with heart tissue in the ventricle in order to sense electrical activity and stimulate and connected to a conductor for transmitting signals to and from the electrode.
2. Description of the Prior Art
Stimulation in both single chamber and dual chamber pacemakers with ventricular stimulation, i.e. VVI and DDD pacemakers, normally takes place in the apex. A conventional pacemaker of this type for treating patients suffering from e.g. bradycardia requires two additional electrodes, each with its own connecting conductor, one of these electrodes being placed in the heart's atrium and the other in the ventricle. The task of the electrodes is to sense electrical activity in the heart and to emit stimulation pulses when spontaneous electrical activity ceases.
One example of an electrode system of this type is described in e.g. U.S. Pat. No. 4,567,901. In this known system, the electrode cable is subdivided in a precurved area into an atrial section and a ventricular section, said sections in turn being curved so the atrial electrode and the ventricular electrode carried by them come to be disposed at the desired sites in the heart when the electrode system is implanted. Such an electrode system with a precurved electrode cable subdivided into two separate sections is a complex system which is difficult to implant in the patient.
There thus exists a need to achieve an Electrode system for DDD pacemakers with the electrode conductors bundled in a single cable.
In attempts to achieve such a simplification of the electrode system, electrode systems have been used previously with the conductors bundled in a single cable, at one end of which an ordinary stimulation and sensing electrode is arranged for implantation in the conventional manner in the ventricle near the apex. This type of electrode system also includes one or a plurality of electrodes arranged on the cable in the atrium of the heart. However, these atrial electrodes are "floating", i.e. they are normally not in direct contact with electrically active tissue, so effective stimulation is impossible, and sensing with the aid of these electrodes is more difficult.
In one attempt to bring the atrial electrode(s) into contact with the heart tissue in the atrium in order to stimulate there, the part of the cable carrying the atrial electrode(s) has been preshaped so the electrode(s) press(es) against the heart tissue in the atrium, cf U.S. Pat. No. 4,154,247. The cable is thus preshaped into a curve, loop or the like so a ring electrode on the cable makes contact with the atrial wall. This preshaping of the cable, however, makes implantation more difficult, and the contact with the atrial wall is unreliable.
Studies have shown that it would be advantageous with pacemakers providing ventricular stimulation to stimulate high up in tile ventricle, e.g. high up in the septum or occasionally, for practical reasons, in the superior part of the outer ventricular wall. Stimulation in the apex has proved to be capable of rapidly resolving acute problems, but some excess mortality has been observed in patients receiving this type of stimulation compared to stimulation in the upper atrial wall. Stimulation high up in the septum has been found to be similar to natural stimulation, since depolarization then comes through the septum and subsequently spreads across the ventricle with more efficient heartbeats as a result.